To mow or to mow less: Lawn mowing frequency affects bee abundance and diversity in suburban yards
Introduction
Bees and other pollinators provide essential ecosystem services in agricultural and pristine landscapes (Gallai et al., 2009; Ollerton et al., 2011), and are experiencing severe declines on a global scale (Vanbergen et al., 2013). Loss and alteration of habitat primarily due to urban development together with the intensification of agricultural practices (e.g., increased applications of pesticides, tilling, monocultures, reduced season-long floral resources) largely contribute to these declines (Goulson, 2013; Harrison and Winfree, 2015; Vanbergen et al., 2013; Winfree et al., 2009). However, recent urban research has documented cities supporting a surprising level of bee richness and abundance (e.g., Fischer et al., 2016; Frankie et al., 2005; Harrison and Winfree, 2015; Matteson et al., 2008; Pardee and Philpott, 2014; Threlfall et al., 2015), suggesting that public parks, ruderal grasslands, meadows, community gardens and flower gardens in private yards have the capacity to serve as bee refugia (Hall et al., 2017). Some cities may even harbor more diverse and abundant populations of native bees compared with nearby forest preserves and other natural systems (Baldock et al., 2015; Fetridge et al., 2008; Winfree et al., 2007). Consequently, green spaces embedded within the urban matrix could mitigate negative aspects of urban development, by providing pollinator and other wildlife habitat (Goddard et al., 2010). However, it is unclear how bees respond to one of the most pervasive urban green spaces, lawns.
Lawns cover >163,000 km2 in the US and include golf courses, athletic fields, commercial and industrial parks and urban and suburban yards (Milesi et al., 2005). High proportions of lawns are located in yards, and serve both social and environmental functions. From a historical and social perspective, the lawn represented a status symbol of upward mobility and more recently, a platform for self-expression of, or projecting adherence to social norms (Nassauer et al., 2009; Robbins, 2007; Robbins and Sharp, 2003). Lawns also provide important ecosystem services. Depending on soil texture, storm water can infiltrate pervious lawns and can serve as a reservoir for some of the run-off (Mueller and Thompson, 2009). Lawns might also mitigate the urban heat island by regulating humidity, particularly when irrigated (Hall et al., 2016). A suburban lawn's capacity for storing carbon (C) and nitrogen (N) can exceed that of non-urban grasslands (Pouyat et al., 2006; Raciti et al., 2008).
In addition to ecological benefits, the intensive management that lawns require can negatively impact urban and suburban ecosystems. Typical lawn management consists of irrigating, applying chemicals and mowing, and is carried out by millions of individual households and neighborhood associations whose actions have ecological and social consequences (Cook et al., 2012). For example, Americans use up to 48 gal of water per day for irrigating lawns and gardens (Environmental Protection Agency; www.epa.gov/waterwise). In arid regions, this kind of water use diminishes scarce natural resources. In addition, fertilizers and other chemical applications can degrade water quality and contaminate groundwater (Law et al., 2004), while gas-powered lawn mower exhaust fumes elevate CO2 emissions (Zirkle et al., 2011). Although not every household irrigates or fertilizes (Polsky et al., 2014), most households mow to conform to societal expectations, city ordinances, and the personal satisfaction of a neat and tidy yard (Robbins, 2007). Many municipalities even enforce ‘weed laws’ to ensure conformity of the lawn ideal by restricting grass height (e.g., a Chicago ordinance prohibits lawn vegetation from exceeding 24.4 cm; Municipal Code of Chicago: §7–28-120). Intensive lawn management requires time and financial commitments, and are often driven by aesthetics and social norms to adhere to ideals of orderly, weed-free, lush carpets of green grass (Jenkins, 1994; Nassauer, 1995; Nassauer et al., 2009; Robbins, 2007).
One of the outcomes of frequent lawn mowing is a simplistic vegetation configuration. Consequently, many ecologists and wildlife organizations have dismissed the habitat potential of lawns, referring to these lawn-dominated yards as ‘sterile environments for biodiversity’ (Gaston et al., 2005: 3342). However, even with it's simplicity, lawns can support rich and diverse plant communities. A survey of 52 residential lawns in Sheffield, UK recorded 159 species of vascular plants (Thompson et al., 2004). However, floral richness and abundance in these lawns might depend on lawn management behaviors and disturbance (Bertoncini et al., 2012; Grime, 1974; Wastian et al., 2016). Research on bees in New York residential yards that had extensive flower gardens showed that frequent lawn mowing (and herbicide application) depleted lawns of floral resources for bees (Fetridge et al., 2008), suggesting that less frequent mowing and avoiding herbicides could have the opposite effect.
Lawns lacking applications of herbicides and other chemicals generally support spontaneous flowers, such as common dandelion Taraxacum officinale (Asteraceae) and white clover Trifolium repens (Fabaceae) (Bertoncini et al., 2012). This has potential habitat implications for bees (Larson et al., 2014) given their dependence on pollen and nectar resources from flowering plants (Frankie et al., 2005). However, frequent (e.g., weekly) lawn mowing generally prohibits plants from flowering (Fetridge et al., 2008). Because declines in native bees and other pollinators are largely caused by habitat loss (Vanbergen et al., 2013), nectar and pollen from these and other ‘weedy’ species have the potential to support bee conservation in urban areas. In this study, we manipulated lawn mowing behaviors in suburban yards to test the hypothesis that decreasing mowing frequency may result in increased lawn floral resources, and in turn, increased bee abundance, bee richness and bee diversity. Testing the effects of alternative lawn care management practices on floral resources may have important implications for bee and other pollinator populations given the cumulative area of lawns in urban and suburban areas in the U.S. and the millions of people that manage these systems.
Section snippets
Study sites
We conducted the study in 16 single-family, owner-occupied suburban yards (sites) in Springfield, Massachusetts, USA. Because we were working with private households, we relied on volunteers that we recruited via a local tree planting organization. Parcels ranged in size between 0.03 and 0.18 ha (typical of medium-density housing stock within Springfield), and houses were built between 1921 and 1957. We required that the yards not be treated with herbicides or irrigated during the study, or
Bee community composition
The effect of mowing on the bee assemblage was evident in total bee abundance and bee richness. We collected a total of 4587 bees representing 93 species during the ten sampling rounds (see Appendix A1 for complete list and associated life history traits including origin, nesting substrate, behavior and body size). Summarized bee abundance and richness in relation to lawn mowing treatments were as follows: weekly mowing = 1425 bees representing 72 species, mowed every two weeks = 1903 bees
Discussion
By manipulating lawn mowing frequency, we established that lawns with the three-week mowing treatment had significantly greater floral abundance than the one or two-week treatments, and that the two-week mowing regime supported the highest bee abundance yet the lowest bee richness and evenness (Fig. 2; Fig. 4b, c, d). With the inclusion of site-level (i.e., yard flowers in planted beds and lawn size) and neighborhood influences (i.e., percent canopy cover) in our models, we accounted for some
Acknowledgements
We appreciate L. Hilberg, S. Clymer, L. McPherson, B. Hodgkins, K. Bordewieck, A. Tonn, G. Hulten, D. Dorval, E. Rosner, and A. Beauchemin for field and lab assistance. C. Neill, K. Nislow, and three anonymous reviewers provided insightful comments on earlier drafts of the manuscript. We thank M. Veit and S. Droege for help identifying bees. D. Bloniarz assisted with household recruitment. We are extremely grateful for the residents in the East Forest Park, 16 Acres and Forest Park
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